Also check out http://www.ticket2ridebc.com for full accom and riding packages.
The cost of flights have gone up in the past 2 years but if you're flexible with dates check out the deals on canadian affair, http://www.canadianaffair.com/jan-sale/
They are generally the cheapest, (or Air Transat, they are the same company).
As well as the park, if you're going for a couple of weeks, get on a shuttle trip to Pemberton, Squamish or North Shore and experience real BC cycling.
You can hire bikes out there, but the cost to take your own on the plane is less than 2 days hire out there, so worth it for the bit of hassle getting your bikes to the air port.
If you're going before mid July, only the lower trails in the bike park will be open. If you go in May and early June what it actually open will depend on how much snow fall they have managed to clear. I have usually done a lot of riding in Pemberton early season as it's more arid up there.
Food and beer is expensive in Whistler, however a few places have specials on throughout the week, so we go around them to eat 'cheaper'.
If you're going to ride the park more than 2 days then the best deal is to buy the 'Triple Play' card, which give you 3 days in the park at a cheaper rate and any further days are just knocked off your CC at a cheaper rate than a day pass. Another trick is to go over to Pinkbike and buy one of their memberships which gives you 1 free days pass, so you will get 4 days on your triple play card.
I bought this book a few years ago that has been my bible ever since and I try and ride something new every visit, (Only missed 3 years since 2005). There's a couple of ace trails on Blackcomb for example that are only a 30min ride / push from the valley.